This application relates to integration of optical waveguiding elements, such as optical fibers and planar waveguides on substrates to form various optical devices, and more particularly, to techniques and devices for coupling optical energy between a fiber and a waveguide.
Optical waves may be transported through optical waveguiding elements or xe2x80x9clight pipesxe2x80x9d such as optical fibers, or optical waveguides formed on substrates. A typical fiber may be simplified as a fiber core and a cladding layer surrounding the fiber core. The refractive index of the fiber core is higher than that of the fiber cladding to confine the light. Light rays that are coupled into the fiber core within a maximum angle with respect to the axis of the fiber core are totally reflected at the interface of the fiber core and the cladding. This total internal reflection provides a mechanism to spatially confine the optical energy of the light rays in one or more selected fiber modes to guide the optical energy along the fiber core. Optical waveguides formed on substrates can also be designed to provide spatial optical confinement based on total the internal reflection. Planar waveguides, for example, may be formed by surrounding a slab or strip of a dielectric material with one or more dielectric materials with refractive indices less than that of the dielectric slab or strip.
Optical fibers may be used in transmission and delivery of optical signals from one location to another in a variety of optical systems, including but not limited to, fiber devices, fiber links and fiber networks for data communications and telecommunications. Optical waveguides on substrates may be used in integrated optical devices where optical elements, opto-electronic elements, or MEMS elements are integrated on one or more substrates.
The guided optical energy in the fiber or waveguide, however, is not completely confined within the core of the fiber or waveguide. In a fiber, for example, a portion of the optical energy can xe2x80x9cleakxe2x80x9d through the interface between the fiber core and the cladding via an evanescent field that essentially decays exponentially with the distance from the core-cladding interface. The distance for a decay in the electric field of the guided light by a factor of e≈2.718 is about one wavelength of the guided optical energy. This evanescent leakage may be used to couple optical energy into or out of the fiber core, or alternatively, to perturb the guided optical energy in the fiber core.
This application includes techniques for providing evanescent optical coupling between a fiber engaged to a first substrate and a waveguide formed in a second substrate. A portion of the fiber is embedded in an elongated groove in the first substrate and is side polished to form an optical coupling port by removing a portion of the fiber cladding. The first and the second substrates are positioned relative to each other so that the coupling port of the fiber is adjacent to the waveguide to allow for evanescent coupling between the fiber and the waveguide. A single fiber may be optically coupled to two or more waveguides through its different coupling ports located in grooves of the first substrate.
An optical grating may be formed to assist the above optical coupling between the fiber and the waveguide. The grating may be formed in the fiber cladding or the fiber core located in the coupling port of the fiber, in the waveguide, or between the fiber and the waveguide.
The fiber may be mounted and engaged to one or more grooves formed in a substrate in a fiber device. One embodiment of the fiber device includes a substrate that is formed with an elongated groove on one substrate surface, and two openings respectively at two ends of the groove formed through the substrate to extend between the two sides of the substrate. An optical fiber is engaged to the substrate by passing through the two openings. The fiber has at least first, second, and third contiguous fiber portions, where the second fiber portion is disposed in the elongated groove on one side of the substrate, and the first and third fiber portions are located on or over the opposite substrate surface. The fiber cladding in the second fiber portion may be at least partially removed to form an optical coupling port for the fiber. According to another embodiment, the fiber device may also be formed in a substrate that includes grooves formed on both opposing sides of the substrate so that two optical coupling ports may be formed in the fiber that are respectively located on two opposite sides of the substrate.